It has long been recognized that difficulty has been encountered in developing a uniform force between members which engage about a central axis along which either a fluid flows or some mechanical movement takes place. These difficulties are particularly apparent with seals, but occur with other devices as well. Examples of sealing problems about axes on which fluids flow are found in valve structures of many types. Examples of other force problems in connection with mechanical movement may be found in rotary joints through which fluid flows, in shaft seals for blocking fluid flow, in pressure vessel closures, and in brakes and the like. It has been seen as desirable to have a capability for linear motion over short distances to apply forces at varying and controlled levels. However, accomplishing this capability while accommodating reliability under extremes of environment and operating conditions has not previously been possible.